A vortex line is a line drawn through a fluid such that it is every way tangent to the rotation vector, which aspect is parallel to the axis of rotation. A collection of vortex lines through a small closed curve defines a vortex tube, which has certain special properties. Vortex lines move with the fluid, while vorticity is a property of the fluid itself and not of the space it occupies. A smoke ring is a natural example of a closed vortex tube.
Vortex tubes have been tried as gas phase refrigerating devices since the 1930s, but only more recently are they proposed as gas-liquid separation and fractionation devices. One approach to a vortex tube air fractionator was described by Suslov et al in Chemical and Petroleum Engineering 16, 507 Sep.-Oct. 1980, "Development and Investigation of a New Type of Air Separation Apparatus".
Certain changes in the 1980 published article are later described in a U.S. Pat. No. 4,531,371 to the same Suslov et al. It teaches using two vortex tubes in series, but it is equivocal as to how much the 0.sub.2 recovery at a given purity will be improved. In any event, using multiple vortex tubes will increase the size and piping complexity of a refrigeration system. Such a trend is an inappropriate way to go for an onboard refrigeration means that is to be made adaptable to hypersonic aircraft applications.
Nonetheless, a vortex tube separator has been perceived as a viable means for producing liquid O.sub.2 during hypersonic flight, predicated on major configurational changes. This auxiliary O.sub.2 source could be applied as the fuel oxidant for boosting the vehicle into space orbit. On board O.sub.2 generation will reduce the take-off weight if the air separation system can be made lightweight enough to meet the cost-benefit standard.
Vortex tube (VT) separators are promising as they lack moving parts, are unaffected by gravity and acceleration changes, and can process large volumes of air per unit volume and weight of apparatus. The typical configuration and flow patterns of a VT separator are shown in FIG. 1.
As seen, the separator consists of an elongated conical chamber with a tangential nozzle entrance. The chamber has a constricted end having an axial gas stream outlet opening, and on the opposing enlarged end by a diffuser ring having a liquid stream outlet. Supplemental to the air feed through the tangential inlet, the art has suggested that a recycled gas may be introduced through a perforated tube disposed along the central axis of the vortex tube.
The tangential introduction of partially condensed air sets up a two-phase vortex flow consisting of an annular film of liquid on the chamber wall and gaseous core. A liquid film is held on the wall by centrifugal forces that far exceed the effect of gravitational acceleration. Thus, operation of the VT is not effected by the changes in orientation or acceleration that will occur during hypersonic flight. As the film moves from the inlet end of the separator to the diffuser, it exchanges mass with the gaseous core and becomes enriched in oxygen. A liquid stream enriched in oxygen is withdrawn through the diffuser ring.
Two zones may be differentiated in the gaseous core. A peripheral zone in which gas flows in the same direction as the liquid film, and a central core zone in which the gas moves back towards the gas outlet, becoming enriched in nitrogen. The vortex flow is accompanied by phase separation and by development of radial and axial temperature gradients.
Still, application of a two-phase VT to air separation has been restricted due to the inability to concurrently obtain both high purity oxygen and high O.sub.2 recovery rates.